1. Field of the Invention
The present invention relates to a thermal printer, and more particularly to the thermal printer which has at least two printing speed modes, that is, a high-speed mode and a low-speed mode to be selectively switched and is capable of variably controlling a time when current flows through each necessary dot heating element contained in a thermal printing head.
2. Description of the Related Art
In general, the normal thermal printer includes a thermal printing head containing dot heating elements disposed in a vertical line. For printing one character, these dot heating elements are selectively heated to form a character as the thermal printing head travels in the printing direction at a predetermined pitch. As such, one character is printed by the dots for character each time the thermal printing head is travelled by the predetermined number of dots. According to this method, some character patterns may allow the same dot heating elements to be kept heating. The surfaces of these heating elements are heated up too much because of the condensed heat thereon. It results in disadvantageously causing the printed characters to have variable density, remarkably lowering character quality and degrading the dot heating elements.
To overcome the disadvantage, the present applicant knows the related thermal printer is designed to continuously manage printing hysteresis and variably control the time when current flows through each necessary dot heating element according to the printing hysteresis. FIG. 1 is a chart illustrating a table for managing the printing hysteresis of the current bit to the fourth previous bit, wherein O denotes printing, X denotes non-printing, and - denotes "Don't Care". FIG. 2 is a chart illustrating pulse widths T1 to T5 defined according to the printing hysteresis illustrated in FIG. 1. It is clearly understood from these charts that as the previous printing is further than the current printing, the larger pulse width is used for the current printing. That is, the larger pulse width can expand the time when current flows through each necessary dot heating element in order to conform with the fact that the longer time interval from the current printing to the next results in the longer cooling time of the dot heating elements. It results in achieving substantially uniform density on printed characters.
In general this kind of thermal printer is designed to switch the printing speed to a high-speed printinq mode or a low-speed mode. The foregoing table corresponds with the high-speed printing mode at which the dot heating elements do not have enough cooling time because of a shorter printing period. In the low-speed printing mode, however, the dot heating elements operated three bits previous have already cooled down because of the foregoing longer printing period. It is, therefore, unnecessary to manage the printing hysteresis before the third previous bit, though, the printing is operated on the table for the high-speed printing mode. It means that when the printing is done before the third previous bit, the time when current flows through each necessary dot heating element is made shorter than a proper time, resulting in making the printed characters thinner in density. Conversely, if the thermal printer provides the table corresponding with the low-speed printing mode, in the high-speed printing mode, excessive energy is applied on the dot heating elements, thereby rapidly degrading the thermal printing head and reducing the life of the head.